Modern boiling water nuclear reactors (BWR's) include in the core region of the reactor a core bypass volume and a fuel channel volume. The fuel channel volume includes bundles of elongated rods or cladding containing the reacting fuel. These bundles are placed within surrounding channels between lower and upper tie plates, and held in a predetermined array by spacers located axially along the bundles. In many such fuel bundle designs, one or more hollow tubes or "water rods" are also positioned in the bundle to provide an additional volume of moderating water. The core bypass volume is exterior of the channels. It is the region in which the control rods control the nuclear reaction and includes additional water moderator for efficient reaction.
Current fuel bundle design has been limited by the need to operate below thermal limits and avoid thermal hydraulic instabilities and coupled nuclear-thermal-hydraulic reactor core instabilities. The stability limits affect the degree to which the fuel assembly can be optimized for minimum fuel cycle costs.
For BWR's having discrete bundles of fuel rods confined within channel enclosed fuel assemblies, improved bundles are disclosed in commonly owned U.S. Pat. Nos. 5,017,332; 5,112,570; and 5,416,812. In accordance with the disclosures in these patents, partial length rods (PLR's) are utilized which extend upwardly from the bottom of the channel (within the boiling region), but only part way to the top of the channel. In other words, these PLR's are axially shortened with respect to the remaining full length rods (FLR's) and are distributed throughout the fuel bundle, with the preferred disposition being in the interior of the bundle, away from the channel walls. The PLR's terminate at a selected spacer located in the two phase flow region of the fuel bundle assembly. Not all PLR's need be of the same length however. For example, some may terminate at one spacer, while others terminate at another spacer, upstream or downstream of the one spacer. Preferably, the length of PLR's is at least one-half of the total height of the fuel bundle.
In commonly owned co-pending U.S. application Ser. No. 08/624,032, filed Nov. 23, 1994, now U.S. Pat. No. 5,598,450, unfueled followers or fuel rod extensions are attached to the tops of the partial length fuel rods so that, geometrically, they behave like the higher flow resistance FLR's.
In those instances where the space above the PLR's has been left vacant to facilitate removal of the PLR's and to provide a free volume so as to allow two phase mixture to bypass fuel rods and spacers and thus reduce two phase pressure drop, "swirlers" have been attached to the fuel bundle spacers for the purpose of redepositing the water droplets in the partial length bypass flow back onto adjacent fuel rods. This configuration, however, still does not allow field removal of the PLR's.
Swirlers have also been disclosed in conjunction with steam vents. Here again, however, PLR's cannot be removed in the field, and the swirlers per se are not only difficult to manufacture, but are difficult to attach to the fuel bundle spacers.
Accordingly, there remains a need for an arrangement which achieves a swirl effect above PLR's in a cost-effective way, and which also allows for field removal of the PLR's.